The objective of this research is to elucidate basic mechanisms by which cells acquire polarity, a process that is of fundamental importance for the development of an organism. How, for example, do cytoskeletal elements become aligned in a manner that allows cell growth and movement to be directional? Bud formation in S. cerevisiae is a simple model for the development of cell polarity. At the beginning of each cell cycle, a single bud emerges at a nonrandom site. Concomitantly, cell cycle, a single bud being delocalized across the cell to being localized to the emerging bud. An understanding of how bud formation is spatially and temporally regulated will depend upon: 1) the identification of those proteins that comprise the earliest structures that mark and nucleate the construction of a bud site, 2) the identification of cell-cycle-specific interactions among these proteins, and 3) the elucidation of the factors that regulate these interactions. As a first step toward addressing these issues, I have used suppressor analysis to identify five new genes that are implicated in bud emergence. RSR1 is a new member of the ras family and is required for proper bud-site selection. BEM1 and BEM2 appear to be required for bud emergence. The functions of MSB1 and MSB2 are not known. The roles of al of these genes will be explored in the following experiments. 1.The localization and order of function of their products will be explored by immunofluorescence microscopy. 2. The physical interactions among thier products will be investigated by determining which ones can be co-purified. 3.The DNA sequence, and null phenotype, of BEM2 will be determined. 4.Genes that regulate, or are targets of, RSR1 will be identified. 5.Genes that are functionally redundant with MSB2 will be sought. 6.As a first step toward understanding how bud formation is prevented from occurring inappropriately, mutants that can form more than one bud at a time will be analyzed.